1. Field of the Invention
The present invention relates to the general field of molten metal baths for the gasification of materials containing hydrogen and carbon.
2. Discussion of the Prior Art
There is a large and growing market for high-pressure, high-purity hydrogen gas and carbon monoxide gas in oil refining, petrochemical and other industrial markets worldwide. The conventional means for the production of these gases are steam-methane catalytic reforming and vapor-phase gasification of hydrocarbons. While not widely practiced, some inventors have described the use of molten metal reactors utilizing a single reaction zone, such as U.S. Pat. Nos. 4,496,369, 4,511,372, 4,574,714 and 4,602,574, to gasify hydrocarbons. By operating in a single reaction zone, all of the above-mentioned gasifiers produce a single mixed-gas product in which the hydrogen and carbon monoxide are combined. If the objective is to produce a hydrogen-rich gas, then it is necessary to add expensive downstream equipment to remove the carbon monoxide or to convert it to hydrogen (through a water-gas shift reaction). Alternatively, if the objective is to produce a carbon monoxide-rich gas, then the hydrogen must be removed by expensive downstream methods.
Some inventors have described the use of molten metal gasifiers which use two or more reaction zones, such that a hydrogen-rich gas can be produced in a first zone and carbon, which dissolves in the molten iron, is removed by reaction with oxygen in a second (or later) zone. Rasor (in U.S. Pat. Nos. 4,187,672 and 4,244,180) describes a hydrocarbon gasification process in which solid hydrocarbons such as coal are lowered onto the surface of a molten iron bath zone in which high temperature cracking of the hydrocarbons into lighter molecular weight materials takes place and residual carbon is dissolved in the molten iron. The gaseous cracked hydrocarbon products are removed via outlets in the shaft through which the feed hydrocarbon solids drops onto the molten iron. The molten iron containing dissolved carbon is transferred to a second molten iron zone in which an oxygen-containing gas is introduced to convert the carbon into carbon monoxide and raise the temperature of the iron for transfer back to the feed zone. The carbon monoxide is further oxidized above the molten iron bath and heat is recovered via a boiler or similar system. Sulfur in the feed is removed via slag formation on top of the molten iron. This process, while able to produce a hydrogen-rich gas separate from the oxidization gases, produces a relatively low-purity hydrogen-rich gas due primarily to the presence of lighter hydrocarbons which result from the cracking of the solid feed. Furthermore, no attempt is made to produce a high-purity carbon monoxide gas product.
Tyrer (in U.S. Pat. No. 1,803,221) and Nixon (in U.K. Patent 1,187,782) describe in general terms two-zone gasifier processes that have the potential to produce a high-purity hydrogen-rich gas by introducing the hydrocarbon feed below the surface of the molten iron, thereby minimizing the production of cracked products. However, by operating at atmospheric pressure, these molten-metal gasifier processes produce hydrogen-rich and carbon monoxide-rich gases at atmospheric pressure, when in fact most industrial processes require that such gases be available at higher pressures, such as 5 to 100 atmospheres absolute or higher. Thus, when using such processes, it is necessary to compress the gases prior to industrial use, which is very expensive.